Add parameter for purging vdir

The new parameter $purge_vdir in apache class gives the possibility to disable the purging of the virtual hosts directory.
Restrict the versions and add 3.1

~> 2.6 will match 2.7
Merge pull request #159 from richardc/patch-1

Restrict the versions and add 3.1
Merge pull request #139 from mjanser/patch-1

Add parameter for purging vdir
Capitalize RedHat to be in-line with actual factor output
Fix vhost defined type unit test context to reflect actual test being performed.

context lists 'Debian' but test is for RedHat.
Merge pull request #171 from wolfspyre/master

Small fix to unit tests
update version to 0.5.1

updating to a real released version, b/c librarian-puppet
blows up with the RC version.